In access control systems roots modulo X are used where X is a composite number having at least two large prime factors. There are such access control systems where the factoring of X is known to some users. One example is the algorithm due to Rabin which is disclosed in "Probabilistic Algorithms", in S. Traub Ed., "Algorithms and Complexity, New Directions and Recent Results", Academic press, New York, 1976, 21-24.